Permanent magnet synchronous motors (PMSM) are widely used in hybrid electric vehicles and battery electric vehicles. Among the permanent magnet synchronous motors, interior permanent magnet (IPM) motors are the most commonly used motors for HEV/BEV applications due to their high power density, high efficiency and wide speed range.
When a hybrid electric vehicle or battery electric vehicle is in an electric mode (i.e., the mode when it is only running the electric motor without the assistance of an internal combustion engine), the vehicle needs to give the driver similar drive performance as compared to conventional vehicles that only use an internal combustion engine. One of the desired features for hybrid electric and battery electric vehicles is to have a coast-down performance similar to that of conventional vehicles. This requires the electric motor to provide certain brake torque to the vehicle when the accelerator pedal is released. In other words, the mechanical power is converted to electric power and fed back to the battery. This is also called coast-down regenerative braking. Regenerative braking is an energy recovery mechanism that slows down a vehicle by converting its kinetic energy into another form—in the case of hybrid electric and battery electric vehicles, the kinetic energy is converted into electrical energy. In conventional braking systems (i.e., for internal combustion engine vehicles), by contrast, excess kinetic energy is converted into heat by friction in the brake linings; therefore, the excess energy is wasted in these vehicles. For hybrid electric and battery electric vehicles, however, the excess energy can be stored in a battery or bank of capacitors for later use.
However, under certain conditions, (e.g., when the state of charge (SOC) of the battery is high or the battery temperature is hot/cold), regeneration current is not allowed back to the battery. Battery state of charge is the equivalent of a fuel gauge for the battery in a hybrid electric or battery electric vehicle, which measures how fully charged the battery is. Thus, when the state of charge of the battery is high or the battery temperature is hot/cold, the amount of power that can be accepted by the battery is met or exceeded. As such, there is the possibility of detrimental effects to the battery if more power is fed back to it.
Under certain conditions such as e.g., when the SOC is nearly full or the battery temperature is high, if coast-down regeneration is not allowed, the electric motor suddenly has to remove all of its braking torque to prevent the current (i.e., energy converted from kinetic energy) from charging the battery. This affects the smoothness of the driving experience as perceived and felt by the driver. This will give the driver inconsistent drive performance when the above conditions exist compared to when they do not. Thus, there is a need to allow regenerative braking in hybrid electric and battery electric vehicles under all circumstances even when the regeneration current cannot be fed back to the battery.